revisit #552
fix missing test/unit in Ruby >= 2.2.0
Workaround for JRuby failure

As of

- https://github.com/jruby/jruby/issues/1243 and
- https://github.com/jruby/jruby/issues/1789,

there might be a regression in JRuby 1.7.x, while JRuby 9000 works
just fine.